Solubility Product (CIE A Level Chemistry)

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22 November 2023

Solubility is defined as the number of grams or moles of compound needed to saturate 100 g of water, or it can also be defined in terms of 1 kg of water, at a given temperature
- For example, sodium chloride (NaCl) is considered to be a soluble salt as a saturated solution contains 36 g of NaCl per 100 g of water
- Lead chloride (PbCl₂) on the other hand is an insoluble salt as a saturated solution only contains 0.99 g of PbCl₂ per 100 g of water

The solubility product (K_sp) is:
- The product of the concentrations of each ion in a saturated solution of a relatively soluble salt
- At 298 K
- Raised to the power of their relative concentrations

C (s) ⇌ aA^x+ (aq) + bB^y- (aq)

K_sp = [A^x+ (aq)]^a [B^y- (aq)]^b

When an undissolved ionic compound is in contact with a saturated solution of its ions, an equilibrium is established
The ions move from the solid to the saturated solution at the same rate as they move from the solution to the solid
- For example, the undissolved magnesium chloride (MgCl₂) is in equilibrium with a saturated solution of its ions

MgCl₂ (s) ⇌ Mg²⁺ (aq) + 2Cl^- (aq)

Equilibria - Equilibrium between Ionic Compound and Saturated Solution, downloadable AS & A Level Chemistry revision notes

When the undissolved MgCl₂ salt comes in contact with its ions in a saturated solution, an equilibrium between the salt and ions is established

K_sp = [Mg²⁺ (aq)] [Cl^- (aq)]²

C (s) ⇌ aA^x+ (aq) + bB^y- (aq)

K_sp = [A^x+ (aq)]^a [B^y- (aq)]^b

Expressing K_sp of ionic compounds

Give the equilibrium expressions, including units, for the solubility products of the following ionic compounds:
1. Ca(OH)₂
2. Fe₂O₃
3. SnCO₃

Answer 1 - Ca(OH)₂:

Step 1 - Write the balanced chemical equation:
- Ca(OH)₂ (s) $⇌$ Ca²⁺ (aq) + 2OH^– (aq)
Step 2 - Write the K_sp expression:
- K_sp = [Ca²⁺ (aq)] [OH^– (aq)]²
Step 3 - Deduce the units:
- K_sp = [mol dm^-3] x [mol dm^-3]²
- K_sp = [mol dm^-3]³
- K_sp = mol³ dm^-9

Answer 2 - Fe₂O₃:

Step 1 - Write the balanced chemical equation:
- Fe₂O₃ (s) $⇌$ 2Fe³⁺ (aq) + 3O^2– (aq)
Step 2 - Write the K_sp expression:
- K_sp = [Fe³⁺ (aq)]² [O²^– (aq)]³
Step 3 - Deduce the units:
- K_sp = [mol dm^-3]² x [mol dm^-3]³
- K_sp = [mol dm^-3]⁵
- K_sp = mol⁵ dm^-15

Answer 3 - SnCO₃:

Step 1 - Write the balanced chemical equation:
- SnCO₃ (s) $⇌$ Sn²⁺ (aq) + CO₃^2– (aq)
Step 2 - Write the K_sp expression:
- K_sp = [Sn²⁺ (aq)] [CO₃²^– (aq)]
Step 3 - Deduce the units:
- K_sp = [mol dm^-3] x [mol dm^-3]
- K_sp = [mol dm^-3]²
- K_sp = mol² dm^-6